The present invention relates to pressure sensing apparatus and, more particularly, to novel apparatus for sensing an average across a transverse plane of an enclosed duct or both total and static pressure of a gas flowing therethrough.
In heating and ventilating systems wherein air flows through enclosed ducts to outlet openings, it is desirable to be able to measure the velocity of air at various points in the system so that proper quantities are delivered through the various outlets. One method of measuring the velocity of air flow is to sense the total pressure using a device such as an impact tube and to sense static pressure through other probes or openings in the duct wall. The differences between the total and static pressures is the velocity pressure. Velocity is calculated by multiplying the square root of the velocity pressure by a constant. An alternate means is a pitot tube, which measures both the total pressure and the static pressure and is connected to a guage in such a way that the static pressure opposes the total pressure. The result is a guage reading corresponding to the velocity pressure. Pressure sensing instruments from which readings of velocity or volume of flow per unit of time may be directly taken are also available.
One problem associated with accurate pressure measurement is that the velocity, and therefore the total and static pressure readings from which velocity pressure is derived, may not be uniform across any given cross-sectional plane of the duct. In order to obtain a representative indication of the average velocity at any particular transverse plane of the duct, it is necessary to take pressure readings at a number of points and average the corresponding velocities.
Some measurement systems are made with a single pressure sensor which is moved to several locations in the area of air flow, and the results are averaged. In other systems, a plurality of sensors are permanently installed at spaced locations in the same transverse plane of the duct, and the results averaged. In some systems of this type, the sensors are connected to a common plenum or manifold, the pressure within which reflects a composite of the several sensors. In these so-called self averaging systems, however, certain design constraints must be observed in order to minimize errors in the readings.
In U.S. Pat. No. 3,685,355 a plurality of total pressure sensors in the form of impact tubes are connected to a common manifold to which a tube leading through the duct wall to the measuring instrument is connected at a point symmetrically arranged with respect to the sensors. The tube leads from one side of the measuring instrument, externally of the duct, through the wall and interior of the duct and is connected to the midpoint of the manifold to which an equal number of symmetrically arranged sensors are connected on each side of the tube connection.
The apparatus of U.S. Pat. No. 3,581,565 provides another example of the self averaging principles of fluid pressure measurement, applied in the same manner as in the previously mentioned patent. Rather than a tube physically extending through the duct wall from the measuring instrument to the midpoint of the manifold, however, the pressure is communicated to the measuring instrument by a hollow sensing tube within the manifold. The sensing tube has an opening at the center of the manifold tube, through which the manifold pressure is communicated to the inside of the sensing tube, which extends through the wall of the duct for connection to the instrument.
In the aforementioned U.S. Pat. No. 3,685,355 the total pressure may be communicated from the duct to either a single manifold extending transversely across the duct, or a plurality of manifolds connected at their centers by a cross manifold to form a common chamber. In either case, a tube from the measuring instrument enters the duct through a wall thereof and extends to a connection with the manifold, or cross manifold, at the geometric center thereof. Static pressure is sensed by a plurality of probes, each extending through the duct wall to a common manifold, exterior of the duct, from which the pressure is communicated from some convenient point to the measuring instrument.
In U.S. Pat. No. 3,581,565, total pressure is always communicated from the duct to a single manifold having therein a sensing tube extending through the duct wall for connection to the measuring instrument. Static pressure is sensed by a single probe extending, from interior mid-point of the duct through the wall thereof for connection to the measuring instrument.
A non-uniform velocity profile across the transverse plane in which the pressures are sensed means that the pressure will be higher at some sensor locations than at others. Thus, air may flow into the probes located at the high pressure points, through the manifold, and out of the probes at the low pressure points. If air flow through the manifold is appreciable, the conditions in an enclosed vessel to which Pascal's law applies will not prevail. That is, change in pressure at some point within the manifold will not be transmitted undiminished throughout the manifold. The pressure within the manifold will vary from a high near the probes sensing high velocities to a low near the probes sensing low velocities. The point at which an equalized pressure would be obtained is indeterminate. Since the velocity profile is variable in typical heating and air conditioning systems, it is not possible to establish a fixed point within an inadequately proportioned manifold, which corresponds to the equalized pressure in any system wherein the sensing apparatus may be employed. Further, without proper proportioning of manifold and probe components, a pressure corresponding to the true average velocity in the duct may not be developed anywhere within the manifold.
Both of the aforementioned patents use the geometric center of the manifold as the point from which manifold pressure is communicated to the measuring instrument, with sensing locations arranged symmetrically on each side of the center point. While the geometric center may represent the best compromise, it is preferable to provide apparatus wherein the static conditions to which Pascal's law applies are more closely approximated within the manifold, thereby making less critical the location of the point within the manifold from which pressure is communicated to the measuring instrument.